1. Field of the Invention
This invention pertains to a pipe gasket having an asymmetric profile which is made by connecting two ends of an extruded elastomeric material.
The gasket of the invention is used in joining pressure pipe and, more particularly, those pipe with a bell on one end and the other end plain.
2. Description of the Prior Art.
Extruding, cutting and splicing rubber strips of a single hardness and symmetric cross section is well known and many o-rings are made by this process. Such o-rings have round, square, rectangular or other symmetric cross-sectional shapes. Extruding, cutting and splicing of rubber strips of asymmetric cross sections which are made for push-on joint pipe gaskets is not known.
Cast iron pressure pipe with a bell on one end and the other plain or bevelled, have, for many years, been joined by utilizing a rubber gasket which is compressed between the inside walls of the bell and the outside wall of the plain or bevelled end of the next pipe in a series of telescoped pipes. The most successful of such systems provides an elongated retainer groove in the bell with a gasket sealing wall as well as throat and wall portions which guide and limit travel of the plain end as it passes through the bell opening and the rubber gasket. Such a pipe joint is described in U.S. Pat. No. 2,953,398 issued Sept. 20, 1960 and U.S. Pat. No. 4,108,481 issued Aug. 22, 1982. Gaskets of this type generally have three essential features, a sealing bulb portion, a heel portion and an inner conical wall.
Since both the pipe bell and plain end may be produced without machining, relatively large variations in as-cast diameters are encountered. The gasket sealing bulb, in turn, is subjected to a wide range of compressions from approximately 2% to 45% of its original thickness. To aid in entry of the plain end into the gasketed bell over the large range of diameters encountered, the inner wall of the gasket is generally made in a conical form which tapers from the mating throat diameter of the bell to the inner sealing bulb diameter of the gasket. To further aid in the assembly, the gasket bulb has a relatively soft Shore A durometer hardness between about 40 and 60.
The retainer heel portion of the gasket is typically produced from a higher hardness compound than the sealing bulb to aid in retention of the gasket during joint assembly and to prevent blowout of the softer bulb portion when the assembled joint is subjected to high internal pressures. The retainer heel portion of a typical gasket has a Shore A durometer hardness between about 75 to 90. The retainer heel portion is generally designed to fit into a retaining groove of the bell section of the enclosing pipe. Known gaskets exhibiting these features are asymmetric in cross-sectional profile.
Gaskets of similar asymmetric cross-sectional profiles have also been produced from single hardness rubber compounds. If the entire gasket is of the harder compound, extremely high assembly forces are required. If the entire gasket is of the softer compound, the gasket is subject to be dislodged during assembly and, in addition, only relatively low internal pressures can be held. Single hardness gaskets are, therefore, normally produced of an intermediate Shore A hardness range from between about 60 and 75. In general, with single hardness gaskets of this type, one or more of the attributes of the dual hardness gasket is diminished.
It has been conventional to manufacture asymmetric profile annular pipe gaskets by the compression molding process. For dual hardness gaskets, a portion of the mold (corresponding to the retainer heel portion of the gasket) is filled with a rubber compound which, when cured, will have a Shore A durometer hardness of between about 75 and 90 and a second portion of the mold (corresponding to the sealing bulb portion of the gasket) is filled with a rubber compound which, when cured, will have a Shore A durometer hardness of between about 40 and 60. The mold is closed and, with suitable pressure and temperature, the two compounds are bonded together and the gasket is cured.
It is also conventional to manufacture asymmetric profile dual hardness gaskets by extruding the uncured compounds together to form a "preprep stock" of the desired profile cross-sectional area. The uncured preprep stock is then placed in a rubber mold for forming, joining and curing. Asymmetric profile dual hardness and single hardness gaskets are made by these and other well-known molding techniques.